Continuing efforts by the art to improve the properties of aluminum base alloys have taken several approaches.
Aluminum-lithium alloys are being developed in order to achieve low density and high elastic modulus which are characteristic of the alloys; see T. H. Sanders and E. S. Balmuth, "Aluminum-Lithium Alloys: Low Density and High Stiffness", Metal Progress, Vol. 113, No. 3, 32, 1978, and E. A. Starke, Jr., T. H. Sanders, Jr., and I. G. Palmer, "New Approaches to Alloy Development in the Al-Li System", J. Metals, 33, 1981, 24. Recently developed powder metallurgy techniques using rapidly solidified particulate are being applied in an effort to overcome problems which have been experienced with conventional ingot cast aluminum-lithium alloys, namely, segregation effects and low fracture toughness. Specifically, the rapid solidification approach is being used to (1) reduce or eliminate segregation, (2) reduce the grain size, (3) extend solid solubility of additional elements, and (4) refine the dispersoid particle size.
The characteristics of aluminum alloys roduced from rapidly solidified powders have been reviewed recently; see, J. R. Pickens, "Aluminum Powder Metallurgy Technology for High Strength Applications", J. Mats. Sci., 16, 1981, 1437 and T. E. Tietz and I. G. Palmer, "Advanced PM Aluminum Alloys", Proceedings 1981 ASM Materials Science Seminar, Advances in Powder Technology, Louisville, Ky., Oct. 1981, p. 189. Discussed in these references are high strength, aluminum-lithium alloys. These alloys typically contain 1 to 3 weight percent lithium. Various additives have been utilized in these alloys to enhance their properties. Zirconium, for example, results in a finer microstructure which helps to disperse slip, with improved ductility and toughness. Alloys containing high concentrations of zirconium, e.g. more than about 0.15 weight percent, require rapid solidification to avoid segregation of zirconium during cooling. Copper and magnesium also are added to aluminum-lithium alloys to improve strength. Rapid solidification for these alloy additions is not normally required for the concentrations of interest.
Although at their current stage of development the Al-Li-Cu-Mg-Zr alloys show improvement in properties compared to conventionally available aluminum alloys, there is still a need for more advanced aluminum alloys with better specific properties for improved structural applications. Desirable improvements include reduced density, higher modulus of elasticity, high ductility and toughness.
Beryllium-aluminum alloys containing 20 to 90 weight percent beryllium have been produced by atomizing a molten solution of aluminum in beryllium from a temperature of approximately 1370.degree. C.; see, McCarthy et al., U.S. Pat. No. 3,644,889. These alloys, containing more than 20 weight percent beryllium, are characterized by a distinctive microstructural appearance in which the beryllium-rich phase is present in the form of generally particulate, irregularly shaped substantially continuous networks which are interspersed by the aluminum-rich phase. These alloys accordingly do not exhibit the fine microstructure features of the aluminum-lithium alloys. U.S. Pat. No. 3,644,889 discloses various strengthening agents for the aluminum phase of the composite alloy; such agents include Mg, Zn, Cu, Li, Ag, Si, Mn, Ti, Zr and others. Lithium is stated as being present up to 5.5 weight percent. Applicants, however, have determined that some of these named strengthening elements do not in fact strengthen the aluminum phase. An example is copper which is soluble in aluminum as well as in beryllium and when added to an alloy of beryllium and aluminum, it preferentially combines with beryllium and does not strengthen the aluminum phase.
Beryllium-aluminum alloys have an undesirable microstructure and are expected to exhibit low fracture toughness. Also, due to their high beryllium content they are very costly.